我只想在进程和进程调用的 dll 之间共享一个静态/全局变量。 exe和dll在同一个内存地址空间。我不希望变量在其他进程之间共享。
问题的详细说明:
假设 a.cpp 中有一个静态/全局变量 x。 exe foo.exe 和 dll bar.dll 都有 a.cpp，所以变量 x 是在两张图片中。
现在，foo.exe 动态加载(或静态)bar.dll。那么问题来了，变量x是不是被exe和dll共享了。
在 Windows 中，这两个人从不共享 x:exe 和 dll 将拥有一个单独的 x 拷贝。但是，在 Linux 中，exe 和 dll 共享变量 x。
不幸的是，我想要 Linux 的行为。我首先考虑在 Windows 上使用 pragma data_seg。但是，即使我正确设置了共享数据段，foo.exe 和 bar.dll 也不会共享 x。回想一下，bar.dll 被加载到 foo.exe 的地址空间中。但是，如果我运行 foo.exe 的另一个实例，则 x 是共享的。但是，我不希望 x 被不同的进程共享。所以，使用 data_seg 失败了。
我可以通过在 exe 和 dll 之间创建一个唯一的名称来使用内存映射文件，我现在正在尝试。
两个问题:
- 为什么 Linux 和 Windows 的行为不同？谁能详细解释一下？
- 在 Windows 上解决此问题最简单的方法是什么？
最佳答案
要获得主程序和 dll 共享相同 x 的 linux 行为，您可以从 dll 或主程序中导出该变量。另一个模块必须导入该变量。
您可以通过使用 DEF 文件 ( see microsoft's documentation ) 来执行此操作，或者通过在变量定义的位置使用 __declspec(dllexport) 和 __declspec(dllimport) 在它使用的任何其他模块中( see microsoft's documentation )。这与 windows 中的模块之间共享任何函数、对象或变量的方式相同。
如果您希望程序在运行时加载库，但主程序可能必须在加载库之前使用变量，程序应该导出变量，而 dll 应该导入它。这里有点鸡和蛋的问题，因为dll依赖于主程序，而主程序又依赖于dll。见 http://www.lurklurk.org/linkers/linkers.html#wincircular
我已经编写了一个示例，说明如何使用 Microsoft 的编译器和 mingw(Windows 中的 gcc)来执行此操作，包括程序和库可以相互链接的所有不同方式(静态，在程序启动时加载的 dll , dll 在运行时加载)
main.h
#ifndef MAIN_H
#define MAIN_H
// something that includes this
// would #include "linkage_importing.h"
// or #include "linkage_exporting.h"
// as appropriate
#ifndef EXPLICIT_MAIN
LINKAGE int x;
#endif // EXPLICIT_MAIN
#endif // MAIN_H
main.c
#ifdef EXPLICIT_DLL
#include "dyn_link.h"
#endif // EXPLICIT_DLL
#include <stdio.h>
#include "linkage_exporting.h"
#include "main.h"
#include "linkage_importing.h"
#include "dll.h"
FNCALL_DLL get_call_dll(void);
int main(int argc, char* argv[])
{
FNCALL_DLL fncall_dll;
fncall_dll = get_call_dll();
if (fncall_dll)
{
x = 42;
printf("Address of x as seen from main() in main.c: %p\n", &x);
printf("x is set to %i in main()\n", x);
fncall_dll();
// could also be called as (*fncall_dll)();
// if you want to be explicit that fncall_dll is a function pointer
printf("Value of x as seen from main() after call to call_dll(): %i\n", x);
}
return 0;
}
FNCALL_DLL get_call_dll(void)
{
#ifdef EXPLICIT_DLL
return get_ptr("dll.dll", "call_dll");
#else
return call_dll;
#endif // EXPLICIT_DLL
}
dll.h
#ifndef DLL_H
#define DLL_H
// something that includes this
// would #include "linkage_importing.h"
// or #include "linkage_exporting.h"
// as appropriate
// declaration of type to hold a
// pointer to the function
typedef void(*FNCALL_DLL)(void);
#ifndef EXPLICIT_DLL
LINKAGE void call_dll(void);
#endif // EXPLICIT_DLL
#endif // DLL_H
dll.c
#ifdef EXPLICIT_MAIN
#include "dyn_link.h"
#endif // EXPLICIT_MAIN
#include <stdio.h>
#include "linkage_importing.h"
#include "main.h"
#include "linkage_exporting.h"
#include "dll.h"
int* get_x_ptr(void);
LINKAGE void call_dll(void)
{
int* x_ptr;
x_ptr = get_x_ptr();
if (x_ptr)
{
printf("Address of x as seen from call_dll() in dll.c: %p\n", x_ptr);
printf("Value of x as seen in call_dll: %i()\n", *x_ptr);
*x_ptr = 31415;
printf("x is set to %i in call_dll()\n", *x_ptr);
}
}
int* get_x_ptr(void)
{
#ifdef EXPLICIT_MAIN
return get_ptr("main.exe", "x"); // see note in dyn_link.c about using the main program as a library
#else
return &x;
#endif //EXPLICIT_MAIN
}
dyn_link.h
#ifndef DYN_LINK_H
#define DYN_LINK_H
// even though this function is used by both, we link it
// into both main.exe and dll.dll as necessary.
// It's not shared in a dll, because it helps us load dlls :)
void* get_ptr(const char* library, const char* object);
#endif // DYN_LINK_H
dyn_link.c
#include "dyn_link.h"
#include <windows.h>
#include <stdio.h>
void* get_ptr(const char* library, const char* object)
{
HINSTANCE hdll;
FARPROC ptr;
hdll = 0;
ptr = 0;
hdll = LoadLibrary(library);
// in a better dynamic linking library, there would be a
// function that would call FreeLibrary(hdll) to cleanup
//
// in the case where you want to load an object in the main
// program, you can use
// hdll = GetModuleHandle(NULL);
// because there's no need to call LoadLibrary on the
// executable if you can get its handle by some other means.
if (hdll)
{
printf("Loaded library %s\n", library);
ptr = GetProcAddress(hdll, object);
if (ptr)
{
printf("Found %s in %s\n", object, library);
} else {
printf("Could not find %s in %s\n", object, library);
}
} else {
printf("Could not load library %s\n", library);
}
return ptr;
}
linkage_importing.h
// sets up some macros to handle when to use "__declspec(dllexport)",
// "__declspec(dllimport)", "extern", or nothing.
// when using the LINKAGE macro (or including a header that does):
// use "#include <linkage_importing.h>" to make the LINKAGE macro
// do the right thing for importing (when using functions,
// variables, etc...)
//
// use "#include <linkage_exporting.h>" to make the LINKAGE macro
// do the right thing for exporting (when declaring functions,
// variables, etc).
//
// You can include either file at any time to change the meaning of
// LINKAGE.
// if you declare NO_DLL these macros do not use __declspec(...), only
// "extern" as appropriate
#ifdef LINKAGE
#undef LINKAGE
#endif
#ifdef NO_DLL
#define LINKAGE extern
#else
#define LINKAGE extern __declspec(dllimport)
#endif
linkage_exporting.h
// See linkage_importing.h to learn how this is used
#ifdef LINKAGE
#undef LINKAGE
#endif
#ifdef NO_DLL
#define LINKAGE
#else
#define LINKAGE __declspec(dllexport)
#endif
构建mingw显式both.sh
#! /bin/bash
echo Building configuration where both main
echo and dll link explicitly to each other
rm -rf mingw_explicit_both
mkdir -p mingw_explicit_both/obj
cd mingw_explicit_both/obj
# compile the source code (dll created with position independent code)
gcc -c -fPIC -DEXPLICIT_MAIN ../../dll.c
gcc -c -DEXPLICIT_DLL ../../main.c
gcc -c ../../dyn_link.c
#create the dll from its object code the normal way
gcc -shared -odll.dll dll.o dyn_link.o -Wl,--out-implib,libdll.a
# create the executable
gcc -o main.exe main.o dyn_link.o
mv dll.dll ..
mv main.exe ..
cd ..
构建 mingw 显式 dll.sh
#! /bin/bash
echo Building configuration where main explicitly
echo links to dll, but dll implicitly links to main
rm -rf mingw_explicit_dll
mkdir -p mingw_explicit_dll/obj
cd mingw_explicit_dll/obj
# compile the source code (dll created with position independent code)
gcc -c -fPIC ../../dll.c
gcc -c -DEXPLICIT_DLL ../../main.c
gcc -c ../../dyn_link.c
# normally when linking a dll, you just use gcc
# to create the dll and its linking library (--out-implib...)
# But, this dll needs to import from main, and main's linking library doesn't exist yet
# so we create the linking library for main.o
# make sure that linking library knows to look for symbols in main.exe (the default would be a.out)
gcc -omain.exe -shared main.o -Wl,--out-implib,main.a #note this reports failure, but it's only a failure to create main.exe, not a failure to create main.a
#create the dll from its object code the normal way (dll needs to know about main's exports)
gcc -shared -odll.dll dll.o dyn_link.o main.a -Wl,--out-implib,libdll.a
# create the executable
gcc -o main.exe main.o dyn_link.o
mv dll.dll ..
mv main.exe ..
cd ..
构建mingw显式main.sh
#! /bin/bash
echo Building configuration where dll explicitly
echo links to main, but main implicitly links to dll
rm -rf mingw_explicit_main
mkdir -p mingw_explicit_main/obj
cd mingw_explicit_main/obj
# compile the source code (dll created with position independent code)
gcc -c -fPIC -DEXPLICIT_MAIN ../../dll.c
gcc -c ../../main.c
gcc -c ../../dyn_link.c
# since the dll will link dynamically and explicitly with main, there is no need
# to create a linking library for main, and the dll can be built the regular way
gcc -shared -odll.dll dll.o dyn_link.o -Wl,--out-implib,libdll.a
# create the executable (main still links with dll implicitly)
gcc -o main.exe main.o -L. -ldll
mv dll.dll ..
mv main.exe ..
cd ..
构建mingwimplicit.sh
#! /bin/bash
echo Building configuration where main and
echo dll implicitly link to each other
rm -rf mingw_implicit
mkdir -p mingw_implicit/obj
cd mingw_implicit/obj
# compile the source code (dll created with position independent code)
gcc -c -fPIC ../../dll.c
gcc -c ../../main.c
# normally when linking a dll, you just use gcc
# to create the dll and its linking library (--out-implib...)
# But, this dll needs to import from main, and main's linking library doesn't exist yet
# so we create the linking library for main.o
# make sure that linking library knows to look for symbols in main.exe (the default would be a.out)
gcc -omain.exe -shared main.o -Wl,--out-implib,main.a #note this reports failure, but it's only a failure to create main.exe, not a failure to create main.a
# create the dll from its object code the normal way (dll needs to know about main's exports)
gcc -shared -odll.dll dll.o main.a -Wl,--out-implib,libdll.a
# create the executable (exe needs to know about dll's exports)
gcc -o main.exe main.o -L. -ldll
mv dll.dll ..
mv main.exe ..
cd ..
构建mingw static.sh
#! /bin/bash
echo Building configuration where main and dll
echo statically link to each other
rm -rf mingw_static
mkdir -p mingw_static/obj
cd mingw_static/obj
# compile the source code
gcc -c -DNO_DLL ../../dll.c
gcc -c -DNO_DLL ../../main.c
# create the static library
ar -rcs dll.a dll.o
# link the executable
gcc -o main.exe main.o dll.a
mv main.exe ../
cd ..
构建 msvc 显式 both.bat
@echo off
echo Building configuration where both main
echo and dll link explicitly to each other
rd /s /q win_explicit_both
md win_explicit_both\obj
cd win_explicit_both\obj
rem compile the source code
cl /nologo /c /DEXPLICIT_MAIN ..\..\dll.c
cl /nologo /c /DEXPLICIT_DLL ..\..\main.c
cl /nologo /c ..\..\dyn_link.c
rem create the dll from its object code the normal way
link /nologo /dll dll.obj dyn_link.obj
rem create the executable
link /nologo main.obj dyn_link.obj
move dll.dll ..\
move main.exe ..\
cd ..
构建 msvc 显式 dll.bat
@echo off
echo Building configuration where main explicitly
echo links to dll, but dll implicitly links to main
rd /s /q win_explicit_dll
md win_explicit_dll\obj
cd win_explicit_dll\obj
rem compile the source code
cl /nologo /c ..\..\dll.c
cl /nologo /c /DEXPLICIT_DLL ..\..\main.c
cl /nologo /c ..\..\dyn_link.c
rem normally when linking a dll, you just use the link command
rem that creates the dll and its linking library.
rem But, this dll needs to import from main, and main's linking library doesn't exist yet
rem so we create the linking library for main.obj
rem make sure that linking library knows to look for symbols in main.exe (the default would be main.dll)
lib /nologo /def /name:main.exe main.obj
rem create the dll from its object code the normal way (dll needs to know about main's exports)
link /nologo /dll dll.obj main.lib
rem create the executable
link /nologo main.obj dyn_link.obj
move dll.dll ..\
move main.exe ..\
cd ..
构建 msvc 显式 main.bat
@echo off
echo Building configuration where dll explicitly
echo links to main, but main implicitly links to dll
rd /s /q win_explicit_main
md win_explicit_main\obj
cd win_explicit_main\obj
rem compile the source code
cl /nologo /c /DEXPLICIT_MAIN ..\..\dll.c
cl /nologo /c ..\..\main.c
cl /nologo /c ..\..\dyn_link.c
rem since the dll will link dynamically and explicitly with main, there is no need
rem to create a linking library for main, and the dll can be built the regular way
link /nologo /dll dll.obj dyn_link.obj
rem create the executable (main still links with dll implicitly)
link /nologo main.obj dll.lib
move dll.dll ..\
move main.exe ..\
cd ..
构建 msvc 隐式.bat
@echo off
echo Building configuration where main and
echo dll implicitly link to each other
rd /s /q win_implicit
md win_implicit\obj
cd win_implicit\obj
rem compile the source code
cl /nologo /c ..\..\dll.c
cl /nologo /c ..\..\main.c
rem normally when linking a dll, you just use the link command
rem that creates the dll and its linking library.
rem But, this dll needs to import from main, and main's linking library doesn't exist yet
rem so we create the linking library for main.obj
rem make sure that linking library knows to look for symbols in main.exe (the default would be main.dll)
lib /nologo /def /name:main.exe main.obj
rem create the dll from its object code the normal way (dll needs to know about main's exports)
link /nologo /dll dll.obj main.lib
rem create the executable (exe needs to know about dll's exports)
link /nologo main.obj dll.lib
move dll.dll ..\
move main.exe ..\
cd ..
构建 msvc static.bat
@echo off
echo Building configuration where main and dll
echo statically link to each other
rd /s /q win_static
md win_static\obj
cd win_static\obj
rem compile the source code
cl /nologo /DNO_DLL /c ..\..\dll.c
cl /nologo /DNO_DLL /c ..\..\main.c
rem create the static library
lib /nologo dll.obj
rem link the executable
link /nologo main.obj dll.lib
move main.exe ..\
cd ..
关于c++ - 在进程和 DLL 之间共享全局/静态变量，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/4911994/